There are many reasons for fitting to a commercial vehicle a weighing system for indicating the state of payload or loading (e.g. payload safety and load optimisation). Commercial motor vehicles are designed to move quantities of materials or goods on public roads. The vehicles are subject therefore to the interests of local, regional or national authorities who are particularly interested in vehicle overloading which may lead to possibly dangerous driving conditions for the operator and other road users. Another interest arises from the perspective of road or bridge damage by prolonged use by overloaded vehicles. The responsible vehicle operator also has a need to ensure that the payload per journey is maximised safely for commercial reasons.
A commercial vehicle is typically made up of three major components for description purposes, namely multi-component suspension assemblies, a chassis and a body. Each suspension assembly itself is made up of a number of suspension components such as axle housings, beams, springs, damping components and bearings. Under loading conditions, these suspension components will move relative to each other and also relative to the chassis or body.
Early weighing systems to indicate axle or vehicle overloading rely on sensors which react to the movement of one of these components relative to another of these components within the suspension assembly or one of these components relative to the chassis or body. These early weighing systems therefore rely on a dynamic device which is attached physically to a number of components that move relative to each other in order to indicate the relative position of one component to another. The dynamic device may be considered as a two (or more) part device and the extent to which parts move (or are affected by the movement of other parts) can be related to the payload weight. One such device is disclosed in U.S. Pat. No. 6,566,864. A conventional weighing system of this type is adversely affected by the very harsh environmental conditions in which it is installed and special measures are required to enable adequate sealing or shielding of the device in service. The weighing system is inherently susceptible to damage from large resilient objects caught or thrown up from a road surface. A device can be damaged if the axle or wheel encounters an over-travel event not typically seen in service (such as travelling on a particularly poor surface or as a result of a vehicle collision).